1. Field of the Invention
The present invention relates to an optical recording medium having a protective film, which is suitable for high-speed reproduction.
2. Description of the Related Art
Ordinary optical recording media have a structure in which a recording film (reflecting film) 3 is formed on a transparent substrate 2 made of, for example, polycarbonate, and a protective film 4 of about 10 xcexcm thickness for protecting the recording film 3 is formed, as shown in FIG. 1.
Here, the protective film 4 is made by applying a protective film forming composition containing an acrylate-type monomer on the recording film 3, and curing it.
For such an acrylate-type monomer, it has been suggested to use a combination of 30 to 60 parts by weight of trifunctional (meth)acrylate ester monomer with low skin irritation (primary irritation index) PII of two or less and with a high transparency of the resulting polymer (for example, a trimethylolpropane triacrylate modified with ethylene oxide), 20 to 50 parts by weight of bifunctional (meth)acrylate ester monomer with a PII of two or less and with which the high liquid viscosity of the modified triacrylate can be adjusted to a viscosity that is suitable for application (for example, hydroxypivalic acid neopentylglycol diacrylate), 5 to 25 parts by weight of a monofunctional (meth)acrylate ester monomer (for example, phenoxyethylacrylate, and 0.01 to 1.0 parts by weight of bifunctional dimethylsiloxane, and 5 to 15 parts by weight of a photoinitiator (Japanese Patent Application Laid-Open No. H7-70472).
Recently, such optical recording media are used not only as audio media, but also as video media, so that they are required to adapt to higher reproduction speeds.
Furthermore, since they are used under rather harsh conditions (for example, in a vehicle at high temperature and humidity in summer), higher durability is required from the optical recording medium.
However, when the speed for reading recorded information is increased in an optical recording media having a protective film made by curing a component including mainly trimethylolpropane triacrylate modified with ethylene oxide with low skin irritation as disclosed in Japanese Patent Application Laid-Open No. H7-70472, there is the problem that the recording signal cannot be reproduced correctly, and errors occur. Moreover, there is the problem that the error frequency increases when the optical recording medium is subjected to high temperatures or high humidities.
It is an object of the present invention to solve the problems of the related art, and to provide an optical recording medium having a protective film made of a protective film forming composition with low skin irritation and small occurrence of errors.
The inventor has found out that for the protective film forming composition for forming the protective film of the optical recording medium, a low level of skin irritation of the entire composition can be maintained by simultaneously using trimethylolpropane triacrylate modified with ethylene oxide (abbreviated as xe2x80x9cEO-modified TMPTAxe2x80x9d in the following), which has low skin irritation, and tripropyleneglycol diacrylate (abbreviated as xe2x80x9cTPGDAxe2x80x9d in the following), and that by combining these at a specified proportion, faster reproduction of the optical recording medium is possible as well as the suppression of errors after subjecting the optical recording medium to high temperatures and humidities, thus conceiving of the present invention.
That is to say, the present invention provides an optical recording medium comprising a substrate, a recording film formed on the substrate, and a protective film formed by applying and curing a protective film forming composition on the recording film, wherein the protective film forming composition comprises 60 to 85 wt % of trimethylolpropane triacrylate modified with ethylene oxide, and 25 to 65 parts by weight of tripropyleneglycol diacrylate per 100 parts by weight of the trimethylolpropane triacrylate modified with ethylene oxide.